1. Field of the Invention
The present invention relates to a digital-to-analog converter circuit which uses a capacitance ratio of capacitors.
2. Description of the Related Art
Conventionally, flat panel display devices such as liquid crystal display devices are commercialized. In particular, a small size, light weight display device is necessary for a portable device, and, for example, a liquid crystal display device is primarily used in a portable phone or the like.
In the liquid crystal display device, because high resolution images are also displayed, an active matrix type liquid crystal display device is used which has a pixel circuit in each display pixel and which can display a high-resolution image.
In a liquid crystal display device, a data voltage corresponding to the brightness is applied to the liquid crystal to realize a display. Therefore, the data signal supplied to each pixel is an analog signal. On the other hand, it is often preferable that the video signal to be supplied to the liquid crystal display device be a digital video signal. In such a case, a digital-to-analog conversion process is applied in the liquid crystal display device and an analog data signal is then supplied to each pixel.
In a digital-to-analog conversion circuit, the power supply voltage is resistively divided and a voltage corresponding to a value of the digital data is generated. However, with an increase in the number of bits of the digital data, the number of stages of resisters is also increased, the voltage value for each stage is reduced, and thus it is difficult to maintain precision for the voltage value. Digital-to-analog converter circuits are known in which a capacitance ratio of capacitors is set based on weights of bits of digital data, a voltage corresponding to “1” and “0” of each bit is set at each capacitor, and a corresponding voltage is set as an output analog voltage by an amount of charge obtained in this manner.
With such a digital-to-analog converter circuit which uses the capacitance ratio, it is possible to easily obtain a relatively small voltage. Therefore, it may be considered preferable to carry out conversion of digital data of a large number of bits by combining the digital-to-analog converter circuit by resistive division and the digital-to-analog converter circuit which uses the capacitance ratio.
Japanese Patent Laid-Open Publication No. 2003-29725, for example, discloses a digital-to-analog conversion of a data signal.
In the digital-to-analog converter circuit which uses the capacitance ratio, it is necessary for charging to the capacitors to be accurately performed in order to improve the precision of the conversion. In a display device, however, video signals which are sequentially supplied must be processed in order, and thus there had been a problem in that the charging process of the capacitor cannot be performed sufficiently accurately.